Use `sudo: true` for 3.7 on xenial.
Use `sudo: true` for 3.7 on xenial.

Not sure why this fixes the build issue, but it was recommended in
travis-ci/travis-ci#9069.

Apparently without `sudo` it appears to try to download
https://s3.amazonaws.com/travis-python-archives/binaries/ubuntu/14.04/x86_64/python-3.7.tar.bz2
which doesn't exist, instead of
https://s3.amazonaws.com/travis-python-archives/binaries/ubuntu/16.04/x86_64/python-3.7.tar.bz2
which does exist.

seomoz/reppy,seomoz/reppy